Steve,
I have not taken apart my Force 12 B-1 baluns but the size and
shape of mine suggest they are likely ferrite beads on a coax center.
Perhaps RG-142 or one of the others with a Teflon dielectric, a classic
ferrite
bead choke balun. I have one on my 6 element 6m Force 12 yagi. I take it the
leads are irretrievable (to reach) to solder back onto the SO-239.
Others have already suggested a replacement. I am only a few steps beyond
your understanding. The ARRL Handbook (mine is c2000) says that âA balanced
antenna should be fed by a balanced feeder to preserve this electrical
symmetry (of your yagi/dipole driven element) with respect to ground,
thereby
avoiding difficulties with unbalanced currents on the line and undesireable
radiation from the transmission line itself. Line radiation can be
prevented
by a number of devices which detune or decouple the line for currents
radiated by the antenna back onto the line that feeds it, greatly reducing
the amplitude of such antenna currents.â
It goes on to say,âa circuit that will isolate the balanced load from the
unbalanced line, while providing efficient power transferââ is called a
balun.
I believe most yagis tend to have a lower impedance than 50 Ohms
(per the Ham Good Book) but I also understand that height above ground,
element sloping, the presence of other parasitic elements all affect this.
So we are not talking about âtransformingâ one impedance to another in
this
case
AND choking coax jacket currents.
The book shows a bead balun attributed to M. Walter Maxwell, W2DU. If I
understand the theory correctly a bead âchokeâ balun would have most of
the
advantages of a wound toroid-type balun without the inductance and stray
capacitance that Jim, K9YC, describes since there is no âcoax outside the
coreâ except at the far ends where the coax enters and leaves.
He does point out in his tutorial that you do end up using a lot
of ferrite (cost) for this (luxury?).
The multiple turns in fewer cores is a lot more efficient use of the
ferrite.
The limiting factor is getting coax that can wind many times in the largest
(2.4 inch) cores, not exceeding the minimum bend radius and compromising the
dielectric but has the specs to work at up to 1.5KW (in some cases or moreâ)
at the frequency in question. Oh, and the jacket must be protected if it
cannot handle the UV. Or you can use larger coax, a few more cores with
fewer turns as Jim suggests.
So I would say you are pretty much âright onâ. It then becomes a question
of whether you want to make an easy to string but expensive bead balun or
a more efficient (cost and perhaps size-wise) and a bit more complex to
make balun with larger cores and multiple turns.
I hope I have it basically correct. I have to keep reading my several books
on Baluns, Un-uns, and transmission lines to keep trying to understand and
hopefully absorb more info. For those of us who were not formally educated
in this the books and a little osmosis are it (with the expert guidance
from those on this reflector). Jimâs tutorial is excellent and I will be
winding more inline chokes with more turns on fewer cores for our
multi-multi
contest station, though we have been using a long bead balun on our 160
vertical.
Good luck! Still learning (I hope).
Aloha,
Kimo Chun, KH7U
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